Presently there exists a multitude of equipment for communicating with general purpose computers. Each piece of equipment has established specifications for the timing, voltage swings and drive capabilities for communicating with the computers. A problem has arisen since the voltage swings for many interfaces are negative, but the widely used logic family is transistor-transistor-logic (TTL) which uses positive voltages. It is then necessary that the positive logic levels must be shifted to negative logic levels whenever new equipment is designed. Previously, this level shifting was accomplished by using discrete devices such as resistors, capacitors, transistors and diodes assembled by hand on a printed circuit board. This method is costly, requires much manual labor, is complex and requires a relatively large volume for packaging. Also there are no provisions for wired-OR operation and suffer damage when the output is accidently short circuited.